The invention relates to a crosslinkable dielectric composition based on polydiene polyol, a polyisocyanate and an inert liquid filler (an insulating liquid) which conserves, before crosslinking, a viscosity which is sufficiently low at a given temperature to allow its use, in particular, for impregnating solid dielectrics such as contained in condensers, cables and measuring transformers.
This composition, which is liquid during the impregnation, becomes an insulating solid after the crosslinking.
The invention also relates to a process for the preparation of the said composition.
The field of application relates most particularly to that of electrical appliances operating by AC or DC voltage and whose insulations are subjected to high electric fields, higher than 10 kV/mm.
For these appliances, one of the main functions of the impregnating liquids is the replacement of all of the air contained in the cavities of the solid insulants.
The presence of air in the solid insulants leads to partial discharges, resulting in the destruction of the dielectric and thus of the appliance.
In certain cases (high-voltage transformers and cables with circulating oil), the insulant also acts as a heat transfer fluid to remove the heat produced in the said appliances by the Joule effect.
However, liquid electric insulants have drawbacks such as the possibility of escaping from the appliances in the case of leaks, with the unfavourable consequences of the risk of the environment being polluted by the liquid and the breakdown of the electrical appliance.
They moreover have the drawback of having a lower breakdown voltage than the solid electric insulants.
Many attempts have been made to impregnate high-voltage insulations with solidifiable liquid systems for all electrical appliances for which the presence of a liquid is not essential in order to remove the heat.
However, these attempts have all been unsuccessful in the field of application concerned, that is to say high-voltage uses with high potential gradient.
As regards solidifiable liquids, two types of compounds have been proposed.
The first type consists of crystallizable organic liquids such as microcrystallized paraffin waxes. For the products of this type, although they possess good dielectric properties, shrinkages are still observed due to the crystallization, which result in the presence of cavities in the impregnated solid insulations.
The second type of solidifiable liquid consists of polycondensable and/or crosslinkable 1- or 2- component systems, filled to a greater or lesser extent, in particular with an inert liquid in order to minimize the shrinkage.
Such systems are described in the Japanese patent application No. 61-197620 published on Sept. 1, 1986.
However, the dielectric properties of these products are mediocre, which prohibits their use in the field of application concerned, namely high-voltage insulations with high potential gradient.
In particular, the angle of loss or dielectric dissipation factor, hereinafter referred to as tangent delta (tan .delta.), is too high.
Thus, in the abovementioned Japanese patent application, the tan .delta. of the products claimed is always higher than 5.times.10.sup.-2 at 20.degree. C., the standard temperature (JIS K-6911).
It may moreover b e noted in this application that the compositions claimed are intended for coating condenser bobbins.